This invention is generally directed to carrier and developer compositions, and more specifically, the present invention relates to developer compositions with coated carrier particles prepared by a dry powder process. In embodiments of the present invention, the carrier particles are comprised of a core with coating thereover generated from a mixture of, for example, three polymers, and wherein the polymers in some embodiments are not in close proximity thereto in the triboelectric series. Moreover, in embodiments the present invention is directed to processes for the preparation of conductive carrier particles, that is with a conductivity of from about 10.sup.-15 to about 10.sup.-6 (ohm-cm).sup.-1, and which carriers possess stable triboelectrical characteristics in the range of from about a negative to a positive 40, 10 to about 35, and preferably in the range of from about 20 to about 25 microcoulombs per gram. Developer compositions comprised of the carrier particles of the present invention are useful in electrostatographic or electrophotographic imaging and printing systems, especially xerographic imaging processes. Additionally, the carrier particles of the present invention are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge on the carrier particles can be preselected depending on the polymer composition applied to the carrier core.
Advantages associated with the present invention include the enablement of obtaining a range of preselected conductivities for carrier particles; permitting the preselection of the triboelectric charge desired on the carrier particles; independently varying and preselecting both conductivity and triboelectric charge; fully and completely coated cores can be obtained wherein the conductive characteristics are not primarily dependent on, or provided by the amount of coating; and long developer life exceeding, for example, 1,000,000 xerographic imaging cycles and wherein the carrier conductivity is from about 10.sup.-15 to about 10.sup.-6 (ohm-cm).sup.-1.
Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or certain conductive carrier components are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions is the appropriate conductivity and triboelectric charging values associated therewith, as it is these values that are of importance for the enablement of continued constant developed images of high quality and excellent resolution.
Carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may comprise various cores, including steel, with a coating thereover of fluoropolymers, and terpolymers of styrene, methacrylate, and silane compounds. A number of carrier coatings, especially carriers which have not been fully coated, can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes, and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles, thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. The aforementioned modification in triboelectric charging characteristics provides developed images of lower quality, and with background deposits.
There are also illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there is disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns, with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320.degree. F. to about 650.degree. F. for a period of about 20 minutes to about 120 minutes enabling the thermoplastic resin particles to melt and fuse on the carrier core. While the developer and carrier particles prepared in accordance with the process of this patent are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances; for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics, and further with regard to the '387 patent, in many situations carrier and developer mixtures with only specific triboelectric charging values can be generated when certain conductivity values or characteristics are contemplated. With the invention of the present application, the conductivity of the resulting carrier particles can be preselected, and moreover, the triboelectric values can be selected to vary significantly, for example from less than -15 microcoulombs per gram to greater than 70 microcoulombs per gram, depending on the polymer mixture selected for affecting the coating processes.
Carrier particles with polymer coatings thereover and which polymers are not in close proximity in the triboelectric series are known, reference U.S. Pat. Nos. 4,937,166 and 4,935,326, the disclosures of which are totally incorporated herein by reference. There are illustrated in these patents carrier particles comprised of a core with a coating thereover comprised of a mixture of a first dry polymer component and a second dry polymer component, which polymer components are not in close proximity in the triboelectric series. These carrier particles can be comprised of known core materials including iron with a dry polymer coating mixture thereover. Subsequently, developer compositions can be generated by admixing the aforementioned carrier particles with a toner composition comprised of resin particles and pigment particles. The percentage of each polymer present in the carrier coating mixture can vary depending on the specific components selected, the coating weight and the properties desired. Generally, the coated polymer mixtures used contain from about 10 to about 90 percent of the first polymer, and from about 90 to about 10 percent by weight of the second polymer. Preferably, there are selected mixtures of polymers with from about 40 to about 60 percent by weight of the first polymer, and from about 60 to about 40 percent by weight of a second polymer. When a high triboelectric charging value is desired, that is exceeding -50 microcoulombs per gram, there is selected from about 90 percent by weight of the first polymer, such as polyvinylidene fluoride; and 10 percent by weight of the second polymer, such as polyethylene. In contrast, when a lower triboelectric charging value is desired, less than about 20 microcoulombs per gram, there is selected from about 10 percent by weight of the first polymer, and 90 percent by weight of the second polymer. Also, there are disclosed in these patents carrier particles of relatively constant conductivities of from between about 10.sup.-15 (ohm-cm).sup.-1 to from about 10.sup.-9 (ohm-cm).sup.-1 at, for example, a 10 volt impact across a 0.1 inch gap containing carrier beads held in place by a magnet; and wherein the carrier particles are of a triboelectric charging value of from -15 microcoulombs per gram to -70 microcoulombs per gram, these parameters being dependent on the coatings selected, and the percentage of each of the polymers used. With the carriers of the present invention, which are preferably essentially completely coated, that is 100 percent coating, the conductivity is provided by the coating polymer, for example four polymers, two polymer pairs, three polymers, and the like, and a number of different conductivities can be achieved in the range of, for example, 10.sup.-6 to about 10.sup.-15 (ohm-cm).sup.-1 ; and further, with the invention carriers there is achievable in embodiments longer lifetimes, superior wear resistance, and excellent resistance to humidity as compared to the carriers of the aforementioned patents.
With further reference to the prior art, carriers obtained by applying insulating resinous coatings to porous metallic carrier cores using solution coating techniques are undesirable from many viewpoints. For example, the coating material will usually reside in the pores of the carrier cores, rather than at the surfaces thereof; and, therefore, is not available for triboelectric charging when the coated carrier particles are mixed with finely divided toner particles. Attempts to resolve this problem by increasing the carrier coating weights, for example, to as much as 3 percent or greater to provide an effective triboelectric coating to the carrier particles necessarily involves handling excessive quantities of solvents, and further usually these processes result in low product yields. Also, solution coated carrier particles when combined and mixed with finely divided toner particles provide in some instances triboelectric charging values which are too low for many uses. The processes, especially powder coating processes of the present invention, overcome or minimize these disadvantages, and further enables developer mixtures that are capable of generating high and useful triboelectric charging values with finely divided toner particles; and also wherein the carrier particles are of a preselected constant conductivity. Moreover, when resin coated carrier particles are prepared by the powder coating process of the present invention, the majority of the coating materials are fused to the carrier surface thereby reducing the number of toner impaction sites on the carrier material. Additionally, there can be achieved with the process of the present invention, independent of one another, desirable triboelectric charging characteristics and conductivity values; that is, for example the triboelectric charging parameter is not dependent on the carrier coating weight as is believed to be the situation with the process of U.S. Pat. No. 4,233,387 wherein an increase in coating weight on the carrier particles may function to also permit an increase in the triboelectric charging characteristics. Specifically, therefore, with the carrier compositions and process of the present invention there can be formulated developers with selected triboelectric charging characteristics and/or conductivity values in a number of different combinations.
Thus, for example, there can be formulated in accordance with the invention of the present application developers with conductivities of from about 10.sup.-15 (ohm-cm).sup.-1 to about 10.sup.-6 (ohm-cm).sup.-1 as determined in a magnetic brush conducting cell; and triboelectric charging values of from about a -40 to a positive 40 microcoulombs per gram; and in embodiments a positive 10 to a positive 30 on the carrier particles as determined by the known Faraday cage technique. Thus, the developers of the present invention can be formulated with constant conductivity values with different triboelectric charging characteristics by, for example, selecting certain carrier coating mixtures.
Other patents of interest include U.S. Pat. No. 3,939,086, which teaches steel carrier beads with polyethylene coatings, see column 6; U.S. Pat. No. 4,264,697, which discloses dry coating and fusing processes; U.S. Pat. Nos. 3,533,835; 3,658,500; 3,798,167; 3,918,968; 3,922,382; 4,238,558; 4,310,611; 4,397,935 and 4,434,220.